1. Field of Invention
This invention relates to techniques for creating predetermined brightness profiles in applications such as backlights for liquid crystal displays and illumination.
2. Description of Related Art
Liquid crystal displays (LCDs) are commonly used in battery operated equipment, such as cell phones, personal digital assistants, and laptop computers, and are becoming popular for desktop and television applications, where they replace bulky CRTs. One embodiment of the present invention deals with a color, transmissive LCD that requires backlighting, where the backlighting may contain red, green, and blue components.
FIG. 1 is a cross-sectional view of a small portion of a prior art, color, transmissive LCD. There are other types of color, transmissive LCD structures. An LCD 10 includes a white light source 12 to provide backlighting for the upper LCD layers. A common source for white light is a fluorescent bulb. Another white light source is a combination of red, green, and blue light emitting diodes (LEDs) whose combined light forms white light. Other white light sources are known. The white light source must provide homogenous light to the back surface of the display. A popular technique for providing such a homogenous white light is illustrated in FIG. 2. White light source 1 is optically coupled to a light guide, such as by optically coupling light source 1 to one or more edges of a sheet of clear plastic 2. The sheet typically has deformities 3 that bend the light approximately normal to the top surface of the sheet so that light is emitted from the top surface. Examples of such deformities include ridges in the bottom surface, reflective particles embedded in the plastic sheet, or a roughening of the bottom surface of the sheet. The deformities cause a quasi-uniform plane of light to be emitted out the front surface of the light guide. A reflector may be placed behind the back surface of the light guide to improve brightness and uniformity.
A brightness enhancing film 13 may be positioned over light source 12. A polarizing filter 14 linearly polarizes the white light. In the embodiment shown in FIG. 1, the polarizing filter 14 is formed in a glass substrate having transparent conductors. Above polarizing filter 14 is a liquid crystal layer 16, and above liquid crystal layer 16 is a glass substrate 18 having transparent conductors. Selected conductors in the glass substrate are energized by display control signals coupled to electrodes 19 and 20. The absence of an electrical field across a pixel area of liquid crystal layer 16 causes light passing through that pixel area to have its polarization rotated orthogonal to the incoming polarization. An electrical field across a pixel area of liquid crystal layer 16 causes the liquid crystals to align and not affect the polarity of the light. Selectively energizing the conductors controls the localized electric fields across the liquid crystal layer 16. Both normally open (white) and normally closed (black) shutters are used in different displays. Instead of a passive conductor array, a transparent thin film transistor (TFT) array may be used, having one transistor for each pixel. TFT arrays are extremely well known.
The light output from glass substrate 18 is then filtered by an RGB pixel filter 22. RGB pixel filter 22 may be comprised of a red filter layer, a green filter layer, and a blue filter layer. The layers may be deposited as thin films. As an example, the red filter contains an array of red light filter areas coinciding with the red pixel areas of the display. The remaining portions of the red filter are clear to allow other light to pass. Accordingly, the RGB pixel filter 22 provides a filter for each R, G, and B pixel in the display.
A polarizing filter 24 only passes polarized light orthogonal to the light output from polarizing filter 14. Therefore, polarizing filter 24 only passes light that has been polarized by a non-energized pixel area in liquid crystal layer 16 and absorbs all light that passes through the energized portions of liquid crystal layer 16. The magnitudes of the electric fields across liquid crystal layer 16 control the brightness of the individual R, G, and B components to create any color. In this manner, any color image may be presented to the viewer by selectively energizing the various conductors.
The desired brightness profile in the LCD is achieved by plastic light guide 2. Plastic light guides such as light guide 2 of FIG. 2 add significant weight to LCD 10. In addition, if light source 1 is red, green, and blue LEDs, light guide 2 must be thick enough to sufficiently mix the light from the individual LEDs, such that the mixed light appears white. Such a thick light guide adds bulk to LCD 10. Further, devices using plastic light guides require many LEDs because of losses in the plastic and at interfaces between the plastic and surrounding materials.